How to make a high-poly monopoly car piece
Actually the solution depends on your current topology, so you could share your file if you want us to give a try.
If I had to do the back of the car from scratch I would begin with a 8th of a subdivided cube that I would stretch and mirror:

Then I would cut the shape of the outgrowth with the knife tool:

Extrude it:

Move the vertices:

Then retopologize:

For the rear, you can create a new edge loop then again, extrude:


So from your current topology:

I would simplify like that, then follow the same techniques:

According to this article, the original car could have been inspired by an old Kurtis Kraft Midget Racer, while others surmise it could be a 1934 Mercedes Grand Prix W25, a Maserati 8CTF, or an Alfa Romero 158.
In my opinion it's a mix but the Mercedes is pretty close:

The Alpha Romeo: